A conventional detecting circuit of a video motion signal comprises a frame memory for storing a video signal to be supplied as a delayed video signal after a period as long as one frame, a difference detecting circuit which comprises a subtracter for subtracting the delayed video signal from an input video signal to generate a frame difference signal, and a low-pass filter for passing through a low frequency component of the difference signal to generate a luminance frame difference signal.
In operation, an input video signal is supplied to the detecting circuit as a video signal of the NTSC mode in which a sum signal of a luminance signal component and a color signal component and a difference signal thereof are supplied interleavingly in every scanning line of the video signals. The input video signal has been digitized by an A/D converter beforehand.
In the difference detecting circuit, a frame difference signal D.sub.j (j is defined as a frame number hereinafter) is defined as the following formula: ##EQU1##
where IN.sub.j corresponds to the input video signal of the frame number j, IN.sub.j-1 corresponds to the delayed video signal supplied from the frame memory, Y.sub.j and Y.sub.j-1 correspond to the luminance signal components, C.sub.j and C.sub.j-1 correspond to the color signal components, and .DELTA.Y corresponds to a luminance frame difference signal.
The luminance frame difference signal .DELTA.Y has no value when a video picture which consists of one frame of video signals is in a still picture mode and has a value in a motion picture mode, on the other hand, the color signal component (C.sub.j +C.sub.j-1) has some value even in the still picture mode. Therefore, in order to extract the luminance frame difference signal .DELTA.Y, the frame difference signal D.sub.j is passed through the low-pass filter to extract a low frequency component thereof which corresponds to the luminance frame difference signal .DELTA.Y. Thus obtained .DELTA.Y corresponds to a video montion signal by which it is detected whether a video picture is in motion or not.
According to the conventional detecting circuit of a video motion signal, however, there is a disadvantage in that separation of the luminance frame difference signal .DELTA.Y and the color signal component (C.sub.j +C.sub.j-1) cannot be carried out perfectly by using only the low-pass filter, because the color signal component (C.sub.j +C.sub.j-1) has a broad distribution which extends to low frequency bands when color phases of the color signal changes largely or the color signal has an early rising distribution as like that made by computer graphic technics, so that the color signal component is included into the output signal of the low-pass filter.
As a result, the detecting circuit detects a signal of the part of the color signal component as a luminance frame differnece signal even in a still picture mode, so that the so called dot crawling may occur in the still picture.
On the other hand, if a low-pass filter having a narrow band characteristic to cut off the color signal component is adopted, the detecting circuit cannot detect the luminance frame difference signal when a value of the luminance signal changes quickly, so that a moving picture may be recognized as a still picture.
As a result, harmful phenomenons such as the cross color, the double pictures, etc. may occur in the still picture.